Async volume migration between backends

Async volume migration between backends

Problem description

Currently if we migrate a volume from one backend to another backend, especially when these two backends are handling two different vendors of arrays, we can’t attach this migrating volume to server until the migration is totally complete.

Both the dd way or driver-specific way will fail if we attach a volume when it is migrating:

  • If we migrate the volume through dd way, the source or target volume is not usable to server.

  • If we migrate the volume through driver-specific way, the migration task is issued to the source backend by scheduler and the volume is owned by the source backend during the whole migration. If we attach this volume to a server when the migration is undergoing, the volume on the source backend(array) will be attached to the server. But after the migration is completed, the source volume on the source backend(array) will be deleted so the previous attachment will actually fails.

Use Cases

Currently cinder supports four cases for volume migration:

  1. Available volume migration between two different backends. These backends could be from one single vendor or different vendors.

  2. Available volume migration between two pools of one single backend.

  3. In-use(attached) volume migration using driver specific way.

  4. In-use(attached) volume migration using Cinder generic migration.

This spec will only focus on case 1, to make an available volume usable(can be attached to a server) immediately after we issue migration, no need to wait until the migration complete. Case 2 is already well handled by most drivers, and if we go into case 2, we won’t go to case 1 again, so this spec will focus on case 1.

Proposed change

In brief, the async migration is to use some features of backend array and I’ve known that these features are already supported by most vendors like EMC VMAX, IBM SVC, HP XP, NetApp and so on. Note that for the backends not support these features, they still can use the existing driver specific migration or host-copy way. We won’t affect anything to the existing routine, just add a new way for developers or users to choose.

These features are:

  • One array can take over other array’s LUN as a remote LUN if these two arrays are connected with FC or iSCSI fabric.

  • We can migrate a remote LUN to a local LUN and meanwhile the remote(source) LUN is writable and readable with the migration task is undergoing, after the migration is completed, the local(target) LUN is exactly the same as the remote(source) LUN and no data will be lost.

To enable one array to take over other array’s volume, we should allow one driver to call other drivers’ interfaces directly or indirectly. These two drivers are from two independent backends, can be managed by one single volume node or two different volume nodes.

To enable an available volume usable(attach to a server) immediately after we issued migration from one backend to another backend, we should ALLOW the migration task to be send to the TARGET backend.

There will be a new interface for driver which will be called ‘migrate_volume_target’. We introduce a new interface instead of use the existing ‘migrate_volume’ interface because there would be lots of differences between them, a significant difference is that the drivers executing the ‘migrate_volume’ interface always take them self as the source backend, but the new interface should take itself as the target backend.

Some change will be made in the volume/ for migrate_volume routine:

  1. If not force_host_copy and new_type_id is None, firstly call source backend driver’s migrate_volume(). If source backend driver’s migrate_volume() return True which means it has migrated successfully, change the “migration_status” to “success” and update the volume, then go to step 3. If source backend driver’s migrate_volume() returned False, go to step 2.

  2. Call target backend driver’s migrate_volume_target() through rpcapi, give a chance for target backend to perform the migration. Give target backend a chance to perform the migration will make the migration more flexible, and is important to enable async migration. migrate_volume_target() should return one more bool value than the migrate_volume() routine to mark the migration is executed synchronously or asynchronously. The whole return value could be: (moved, migration_async, model_update). If migrate_volume_target() returns moved as False which means driver can’t perform the migration, we will go to _migrate_volume_generic() as usual to perform host-copy migration. If migrate_volume_target() returns moved as True and async as False, change the “migration_status” to “success” and update the volume in db, then go to step 3. If migrate_volume_target() returns moved as True and async as True, change the “migration_status” to “migrating_attachable” and update the volume in db, then just go to step 3. “migrating_attachable” means this volume is migrating and is safe to attach it to a server, and end users can check if the volume is attachable to use while it is migrating by “cinder show” command. Note that driver developer should make sure the volume on the corresponding backend is safe to read/write and no data corruption will occur while the migration is undergoing before implementing the migrate_volume_target() interface.

  3. Update the to the target host and update the db with model_update. Now the volume is usable for server to perform read/write on it. If migrate_volume_target() returns async as False, the whole migrate_volume() routine is end now. If migrate_volume_target() returns True, go to step 4.

  4. Call target backend driver’s complete_migration_target() to monitor the undergoing migration, and do some cleaup after the migration is totally completed.

  5. Update “migration_status” to “success” and end the whole migrate_volume() routine.


Let users wait a long time before the volume is usable for a server until the whole migration is totally complete.

REST API impact


Data model impact


Security impact


Notifications impact

Currently no direct notifications. But users can know that the async migration is started when the “migration_status” changed to “migrating_attachable” and is finished when the “migration_status” changed to “success”.

For other impact, like what operations are permitted and what operations are not permitted, I think it’s same like the existing migration.

Other end user impact

After issued “cinder migrate” command, end users can check the “migration_status” by “cinder show” command. If “migration_status” is “migrating”, this volume is probably not safe to attach, and if “migration_status” is “migrating_attachable” or “success”, we can attach it safely. But one thing we should let end user know is that if “migration_status” is “migrating_attachable”, the volume is safe to attach but the performance of the volume may not as good as other volumes for the moment before the undergoing migration is done.

Performance Impact

As we know, driver assisted migration is mostly more efficient than host copy, assuming that there is no read-through along with the migration.

If we attach the migrating volume and do read-through operations on the volume, the performance of the read/write is surelly not so good as the direct read/write. As far as I know, for the OLTP workload, the performance may decrease no more than 15 percent.

Other deployer impact

If deployers want to use the async feature, they should make sure the backend driver supports this feature, and make sure these backends are connected to each other.

Developer impact

Driver developers should implement two new interfaces: “migrate_volume_target” and “complete_migration_target”.

If drivers won’t support this feature, driver devlopers needn’t do anything. We won’t break down any driver or any existing function.



Primary assignee:

Wilson Liu <>

Work Items

  • Implement the proposed change




  • Unit-tests should be implemented

Documentation Impact




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